With the development of telecommunication systems, the use of facsimile (synonymously referred to as “fax”) transmissions to carry data is becoming increasingly widespread. Fax machines, employing a wide array of technologies and various coding schemes and protocols, are now an increasingly integral and ubiquitous part of the modern business and personal world. There is every reason to believe that this trend will continue.
The latest facsimile protocol, International Telecommunications Union Recommendation (ITU) V.34 half-duplex, uses phase and amplitude modulated “symbols” to transmit and receive the data of a facsimile image. When the received symbols are clean and distinguishable, they can be correctly identified and the fax image data they carry decoded.
ITU V.34 protocols calls for time division multiplexing between primary (user data) channel and control channel transmission. The primary channel employs high speed ITU V.34 modulation to transmit data of fax images, while the control channel uses a lower speed, full duplex, ITU V.21 modulation. The control channel is employed to communicate control information, such as symbol speed and density, that will subsequently be used in the primary channel (V.34) communication.
Fax machines often use an equalizer to adjust incoming symbols such that they can be properly identified and decoded. Since the primary channel transmission is not present during the control channel transmission period, the equalizer of the receiving fax must be retrained before every transfer back from the control channel to the primary channel. If the equalizer is not trained before the fax receiver enters into the primary channel, the received symbols will not be correctly identified, and erroneous fax data will result. Typically, the equalizer must be retrained with every page of faxed data sent.
Certain techniques have been developed to help decrease the necessary equalizer retraining time. For instance, it is advantageous for a fax machine to sample the incoming symbols of the primary channel transmission at the same time of a symbol period as when the equalizer was trained during a “phase three” initialization period of the fax machine start up. If the equalizer of the receiving fax machine uses its phase three symbol sampling coefficients (generated during the phase three initialization process) as a factor in retraining the equalizer before the fax receiver enters into the primary channel, the time necessary for equalizer retraining can be significantly reduced.
Nonetheless, the phase three coefficients should accurately reflect the actual time in a symbol sampling period that is used in the primary channel reception by the receiving fax machine. If, for some reason, the fax machine is not sampling the incoming symbols of the primary channel transmission at the same rate as the incoming symbols were sampled during the phase three initialization of the fax machine, the incoming symbols may be read by the equalizer as blurry and indistinct.
To combat this problem, a signal analysis technique known as “timing jamming” may be advantageously employed. Those skilled in the art are familiar with timing jamming. After employing the technique of timing jamming, the receiving fax machine may create “clean” symbols for use by its equalizers. The equalizer can then advantageously decode, with the aid of their phase three coefficients, the incoming symbol data.
However, after using the timing-jamming technique, the symbols may still be rotated by an arbitrary offset angle, and thus may be misinterpreted by the equalizer, leading to erroneous output. Further, since the output of the equalizer may be used to retrain or fine tune the equalizer at the next transfer from the control channel to the primary channel, these output errors can result poor equalizer performance. The retrained equalizer's erroneous coefficients may significantly differ from the coefficients obtained during the phase three initialization period of the receiving fax machine. The phase three coefficients may be of little use in helping the equalizer to interpret any incoming symbols, which could greatly increase the time necessary for equalizer retraining and reinitialization for symbol interpretation after primary channel transfer.
Therefore, what is needed in the art is a technique, involving little retraining, to determine the phase angle by which incoming symbols should be offset to allow them to be properly interpreted by the receiving fax machine equalizers.